UX: ping --ip help text misleading; use MarkFlagsOneRequired for operatorsInfo

[olegshmuelov]

Problem

The --ip flag help for ping is misleading:

Operator endpoint(s) to ping (https://host:port). If empty, pings all
operators from operatorsInfo

This implies --ip is primary and --operatorsInfo is a fallback. In
reality, --operatorsInfo (or --operatorsInfoPath) is required for
identity verification; --ip is an optional filter to target a subset of
operators.

Users running ssv-dkg ping --ip https://host:3030 without operatorsInfo
get a runtime error ("operators info should be provided..."). Functional but
confusing — help text doesn't reflect the new requirement introduced in
v3.1.0.

Proposed fixes

1. Update --ip description (cli/flags/healthcheck.go:8)

"Optional filter: specific operator endpoints (https://host:port) to ping. "
"Requires --operatorsInfo or --operatorsInfoPath. "
"If omitted, pings all operators from operatorsInfo."

2. Use cobra's MarkFlagsOneRequired

Move the runtime check to parse-time via cobra:

cmd.MarkFlagsOneRequired("operatorsInfo", "operatorsInfoPath")
cmd.MarkFlagsMutuallyExclusive("operatorsInfo", "operatorsInfoPath")

This replaces the manual check in cli/initiator/initiator_health_check.go
and surfaces the error before the binary does any setup — better UX.